Catalytic reforming is well known in the petroleum industry and involves the treatment of naphtha fractions to improve octane rating by the production of aromatics. The more important hydrocarbon reactions which occur during the reforming operation include the dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, and dehydrocyclization of acyclic hydrocarbons to aromatics. A number of other reactions also occur, including the dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butane. It is important to minimize hydrocracking reactions during reforming as they decrease the yield of gasoline boiling products and hydrogen.
Because there is a demand for high octane gasoline, extensive research has been devoted to the development of improved reforming catalysts and catalytic reforming processes. Catalysts for successful reforming processes must possess good selectivity. That is, they should be effective for producing high yields of liquid products in the gasoline boiling range containing large concentrations of high octane number aromatic hydrocarbons. Likewise, there should be a low yield of light gaseous hydrocarbons. The catalysts should possess good activity to minimize excessively high temperatures for producing a certain quality of products. It is also necessary for the catalysts to either possess good stability in order that the activity and selectivity characteristics can be retained during prolonged periods of operation; or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalytic reforming is also an important process for the chemical industry. There is an increasingly larger demand for aromatic hydrocarbons for use in the manufacture of various chemical products such as synthetic fibers, insecticides, adhesives, detergents, plastics, synthetic rubbers, pharmaceutical products, high octane gasoline, perfumes, drying oils, ion-exchange resins, and various other products well known to those skilled in the art.
An important technological advance in catalytic reforming has recently emerged which involves the use of large-pore zeolite catalysts. These catalysts are further characterized by the presence of an alkaline earth metal and are charged with one or more Group VIII metals. This type of catalyst has been found to advantageously provide higher selectivity and longer catalytic life than those previously used.
Having discovered selective catalysts with acceptable cycle lives, successful commercialization seemed inevitable. Unfortunately, it was subsequently discovered that the highly selective, large pore zeolite catalysts containing a Group VIII metal were unsually susceptible to sulfur poisoning. See U.S. Pat. No. 4,456,527, which is hereby incorporated by reference for all purposes. Ultimately, it was found that to effectively address this problem, sulfur in the hydrocarbon feed should be at ultra-low levels, preferably less than 100 parts per billion (ppb), more preferably less than 50 ppb to achieve an acceptable stability and activity level for the catalysts.
While low sulfur systems were initially effective, it was discovered that a shut down of the reactor system can be necessary after only a matter of weeks. The reactor system of one test plant had regularly become plugged after only such brief operating periods. Not surprisingly, the plugs were found to be those associated with coking. However, although coking is a common problem in hydrocarbon processing, the extent and rate of coking associated with this particular system far exceeded any expectation. See U.S. Ser. No. 666,696, entitled "Low Sulfur Reforming Processes, " filed Sep. 3, 1991 (and now abandoned), which is hereby incorporated by reference for all purposes. That application discloses a reactor system of improved resistance to carburization and metal dusting under conditions of low sulfur. At least a portion of the reactor system in contact with the hydrocarbons is tin.